Boris Johnson, the mayor of London, was branded 'naive' yesterday after raising the prospect of an amnesty for illegal immigrants, a move that puts him in open opposition to Conservative leader David Cameron.Johnson's plan to study the potential benefits of an amnesty was attacked by the government's Phil Woolas, who said it could lead to more people being exploited by traffickers. It was also ruled out by Johnson's own party, as shadow home secretary Dominic Grieve warned that an amnesty would be 'massively counter-productive'.Woolas, the Immigration Minister, said: 'I think this is naive of the mayor. His comments might start with the best of intentions but will lead to more people-traffickers making more money and exploiting more vulnerable individuals.'The UK Border Agency is committed to stopping illegal migration. We are putting in place the biggest shake-up of the immigration system for 45 years and are seeing the results. We are putting more resources into expelling foreign lawbreakers and last year we removed one person every eight minutes.'Johnson's comments risk opening a rift between himself and Cameron, with whom he clashed on the issue of an amnesty earlier this year. It will do nothing to quell speculation that the maverick mayor is positioning himself as a rival to his party leader, particularly at a moment of Tory weakness in the polls.Grieve told The Observer yesterday: 'Our policy remains unchanged. We're not in favour of having an amnesty, because the evidence is overwhelming that amnesties encourage more people to come into this country as illegal immigrants. We've considered this very carefully and remain firmly of the opinion that to have an amnesty at the present time with the current state of our borders would be potentially massively counter-productive.'If the situation arises where we have complete control of our borders and are satisfied that any future inflow of illegal immigrants has been properly curbed, obviously one must always keep open reviews of policy in that context. But at present our borders are not secure in any way and it would simply encourage massive further immigration.'He played down suggestions that Johnson is seeking to undermine the party leadership: 'He's the mayor of London and is entitled to express his view, particularly in the context of the problems he has to face. It's an interesting contribution to the debate, but doesn't change my view on the matter.'Of the 700,000 immigrants thought to be working illegally in the UK, about 400,000 are in London. Johnson said that allowing long-term illegal immigrants to earn the right to stay in Britain would see 'hugely increased' tax revenues. Speaking on Channel 4 News, the mayor said: 'What I want to do is commission a study by my own economics team here at the Greater London Authority into the possibility. We want to look in detail at what the economic impact of such an earned amnesty system would be.'He acknowledged illegal immigrants had broken the law and should 'in principle' be deported, but added: 'Unfortunately it's just not going to happen.'Johnson raised the issue during his campaign for City Hall in April. Cameron was quick to express his disagreement, saying: 'The problem with amnesties is that people expect another one in the future.' The pair also clashed in the summer when Johnson described his leader's claim that Britain is a broken society as 'piffle'.Sir Andrew Green, chairman of MigrationwatchUK, said the amnesty proposal was 'unbelievably irresponsible' and would 'cost the taxpayer at least £500m a year'.